diff --git a/CHANGELOG.md b/CHANGELOG.md
index f48e96495c..77e942144f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,6 +20,7 @@
 
 - Add location information to duplicate type definition error messages. https://github.com/rescript-lang/rescript-compiler/pull/6199
 - Replace normal module errors with Super_error module, and clean up Super_error. https://github.com/rescript-lang/rescript-compiler/pull/6199
+- `Js.Json.t`, `Js.null` and `Js.nullable` are now untagged variants representing their runtime values, instead of abstract types. https://github.com/rescript-lang/rescript-compiler/pull/6218
 
 # 11.0.0-alpha.5
 
diff --git a/jscomp/others/js.ml b/jscomp/others/js.ml
index 90b8c28833..39ae6a3a30 100644
--- a/jscomp/others/js.ml
+++ b/jscomp/others/js.ml
@@ -86,7 +86,10 @@ end
 
 (**/**)
 
-type +'a null
+type +'a null =
+  | Value of 'a
+  | Null [@as null]
+[@@unboxed]
 (**
   Nullable value of this type can be either null or 'a. This type is equivalent to Js.Null.t.
 *)
@@ -96,7 +99,12 @@ type +'a undefined
   A value of this type can be either undefined or 'a. This type is equivalent to Js.Undefined.t.
 *)
 
-type +'a nullable
+type +'a nullable =
+  | Value of 'a
+  | Null [@as null]
+  | Undefined [@as undefined]
+[@@unboxed]
+
 (**
   A value of this type can be undefined, null or 'a. This type is equivalent to Js.Null_undefined.t.
 *)
diff --git a/jscomp/others/js_json.ml b/jscomp/others/js_json.ml
index f4d369f33c..d135cdad32 100644
--- a/jscomp/others/js_json.ml
+++ b/jscomp/others/js_json.ml
@@ -24,7 +24,15 @@
 
 (** Efficient JSON encoding using JavaScript API *)
 
-type t
+type t =
+  | False [@as false]
+  | True [@as true]
+  | Null [@as null]
+  | String of string
+  | Number of float
+  | Object of t Js.Dict.t
+  | Array of t array
+[@@unboxed]
 
 type _ kind =
   | String : Js_string.t kind
diff --git a/jscomp/others/js_json.mli b/jscomp/others/js_json.mli
index 5b0f403ccc..87b69ba7dd 100644
--- a/jscomp/others/js_json.mli
+++ b/jscomp/others/js_json.mli
@@ -30,7 +30,15 @@
 
 (** ## Types *)
 
-type t
+type t =
+  | False [@as false]
+  | True [@as true]
+  | Null [@as null]
+  | String of string
+  | Number of float
+  | Object of t Js.Dict.t
+  | Array of t array
+[@@unboxed]
 (** The JSON data structure *)
 
 (** Underlying type of a JSON value *)
diff --git a/jscomp/others/js_null.ml b/jscomp/others/js_null.ml
index d9fa7dfd88..19e2fd7915 100644
--- a/jscomp/others/js_null.ml
+++ b/jscomp/others/js_null.ml
@@ -25,7 +25,10 @@
 (** Provides functionality for dealing with the `'a Js.null` type *)
 
 
-type + 'a t = 'a Js.null
+type + 'a t = 'a Js.null =
+  | Value of 'a
+  | Null [@as null]
+[@@unboxed]
 
 external to_opt : 'a t -> 'a option = "#null_to_opt"
 external toOption : 'a t -> 'a option = "#null_to_opt"
diff --git a/jscomp/others/js_null.mli b/jscomp/others/js_null.mli
index c3e6a24f77..d7976f50c9 100644
--- a/jscomp/others/js_null.mli
+++ b/jscomp/others/js_null.mli
@@ -26,7 +26,10 @@
 
 (** Provides functionality for dealing with the `Js.null('a)` type *)
 
-type +'a t = 'a Js.null
+type +'a t = 'a Js.null =
+  | Value of 'a
+  | Null [@as null]
+[@@unboxed]
 (** Local alias for `Js.null('a)` *)
 
 external return : 'a -> 'a t = "%identity"
diff --git a/jscomp/others/js_null_undefined.ml b/jscomp/others/js_null_undefined.ml
index 415f966642..d91319e0fa 100644
--- a/jscomp/others/js_null_undefined.ml
+++ b/jscomp/others/js_null_undefined.ml
@@ -24,7 +24,12 @@
 
 (** Contains functionality for dealing with values that can be both `null` and `undefined` *)
 
-type + 'a t = 'a Js.nullable
+type + 'a t = 'a Js.nullable = 
+  | Value of 'a
+  | Null [@as null]
+  | Undefined [@as undefined]
+[@@unboxed]
+
 external toOption : 'a t -> 'a option = "#nullable_to_opt"
 external to_opt : 'a t -> 'a option = "#nullable_to_opt"
 external return : 'a -> 'a t = "%identity"
diff --git a/jscomp/others/js_null_undefined.mli b/jscomp/others/js_null_undefined.mli
index b81a928e62..9f5d8655e5 100644
--- a/jscomp/others/js_null_undefined.mli
+++ b/jscomp/others/js_null_undefined.mli
@@ -24,7 +24,11 @@
 
 (** Contains functionality for dealing with values that can be both `null` and `undefined` *)
 
-type +'a t = 'a Js.null_undefined
+type +'a t = 'a Js.nullable = 
+  | Value of 'a
+  | Null [@as null]
+  | Undefined [@as undefined]
+[@@unboxed]
 (** Local alias for `Js.null_undefined('a)`. *)
 
 external return : 'a -> 'a t = "%identity"
diff --git a/jscomp/others/release.ninja b/jscomp/others/release.ninja
index a6335e32c2..226767ce46 100644
--- a/jscomp/others/release.ninja
+++ b/jscomp/others/release.ninja
@@ -32,7 +32,7 @@ o others/js_float.cmi others/js_float.cmj : cc others/js_float.ml | others/belt_
 o others/js_global.cmi others/js_global.cmj : cc others/js_global.ml | others/belt_internals.cmi others/js.cmi $bsc
 o others/js_int.cmi others/js_int.cmj : cc others/js_int.ml | others/belt_internals.cmi others/js.cmi $bsc
 o others/js_json.cmj : cc_cmi others/js_json.ml | others/belt_internals.cmi others/js.cmi others/js.cmj others/js_array2.cmj others/js_dict.cmj others/js_json.cmi others/js_string.cmj others/js_types.cmj $bsc
-o others/js_json.cmi : cc others/js_json.mli | others/belt_internals.cmi others/js.cmi others/js_dict.cmi others/js_null.cmi others/js_string.cmj others/js_types.cmi $bsc
+o others/js_json.cmi : cc others/js_json.mli | others/belt_internals.cmi others/js.cmi others/js.cmj others/js_dict.cmi others/js_null.cmi others/js_string.cmj others/js_types.cmi $bsc
 o others/js_list.cmj : cc_cmi others/js_list.ml | others/belt_internals.cmi others/js.cmi others/js_array2.cmj others/js_list.cmi others/js_vector.cmj $bsc
 o others/js_list.cmi : cc others/js_list.mli | others/belt_internals.cmi others/js.cmi $bsc
 o others/js_map.cmi others/js_map.cmj : cc others/js_map.ml | others/belt_internals.cmi others/js.cmi $bsc
diff --git a/jscomp/runtime/js.ml b/jscomp/runtime/js.ml
index dacc4aceca..38c7d9949e 100644
--- a/jscomp/runtime/js.ml
+++ b/jscomp/runtime/js.ml
@@ -57,7 +57,10 @@ end
 (**/**)
 
 
-type + 'a null
+type + 'a null =
+  | Value of 'a
+  | Null [@as null]
+[@@unboxed]
 (** nullable, value of this type can be either [null] or ['a]
     this type is the same as type [t] in {!Null}
 *)
@@ -66,7 +69,11 @@ type + 'a undefined
 (** value of this type can be either [undefined] or ['a]
     this type is the same as type [t] in {!Undefined}  *)
 
-type + 'a nullable
+type + 'a nullable =
+  | Value of 'a
+  | Null [@as null]
+  | Undefined [@as undefined]
+[@@unboxed]
 (** value of this type can be [undefined], [null] or ['a]
     this type is the same as type [t] n {!Null_undefined} *)
 
diff --git a/jscomp/test/custom_error_test.js b/jscomp/test/custom_error_test.js
index 93b4a7a751..5d2c407636 100644
--- a/jscomp/test/custom_error_test.js
+++ b/jscomp/test/custom_error_test.js
@@ -2,7 +2,6 @@
 'use strict';
 
 var Js_exn = require("../../lib/js/js_exn.js");
-var Caml_option = require("../../lib/js/caml_option.js");
 var Caml_js_exceptions = require("../../lib/js/caml_js_exceptions.js");
 
 function test_js_error(param) {
@@ -18,7 +17,7 @@ function test_js_error(param) {
     }
     throw err;
   }
-  return Caml_option.some(e);
+  return e;
 }
 
 function test_js_error2(param) {
@@ -48,12 +47,12 @@ function example1(param) {
     }
     throw err;
   }
-  return Caml_option.some(v);
+  return v;
 }
 
 function example2(param) {
   try {
-    return Caml_option.some(JSON.parse(" {\"x\"}"));
+    return JSON.parse(" {\"x\"}");
   }
   catch (raw_exn){
     var exn = Caml_js_exceptions.internalToOCamlException(raw_exn);
diff --git a/jscomp/test/js_json_test.js b/jscomp/test/js_json_test.js
index 4248f81147..8068a48e4c 100644
--- a/jscomp/test/js_json_test.js
+++ b/jscomp/test/js_json_test.js
@@ -81,7 +81,7 @@ add_test("File \"js_json_test.res\", line 22, characters 11-18", (function (para
                   _0: false
                 };
         }
-        var ty2 = Js_json.classify(Caml_option.valFromOption(v$1));
+        var ty2 = Js_json.classify(v$1);
         if (typeof ty2 !== "object") {
           return {
                   TAG: "Ok",